An n-type crystal is mainly used in switching devices for power application such as IGBT (insulated gate bipolar transistor) or the like. Conventionally, EPW (an epitaxial wafer) and FZ-PW (a wafer produced by the FZ method), whose resistivity can be relatively easily controlled, have been used. Unfortunately, EPW is expensive since an additional step (an epitaxial growth step) is included compared with CZ-PW (a wafer produced by the CZ method), and the FZ method has a difficulty in increasing a crystal diameter. Therefore, use of CZ-PW on which no epitaxial layer is stacked has attracted attention. However, a silicon single crystal produced by the CZ method causes a segregation phenomenon, so that it is difficult to uniform its resistivity distribution in an axial direction (a pulling axial direction).
To solve this problem, Patent Documents 1 and 2 disclose methods in which a primary dopant and a secondary dopant having an opposite polarity and a small segregation coefficient relative to the primary dopant are added (i.e., counter doping). Such methods can improve the axial resistivity distribution of CZ single crystals. It should be noted that a dopant mostly used for producing n-type crystals is phosphorus (P), whose segregation coefficient is about 0.35. Meanwhile, an element having an opposite polarity and a smaller segregation coefficient than that of phosphorus (P) is, for example, Ga, In, and Al. These elements are heavy metals, and it is reported that when such elements are contained in, for example, an oxide film, the oxide film is degraded in electrical characteristics. Effects of these elements on device characteristics are not well known. Therefore, B (boron), which is a mainly used p-type dopant and widely used for producing devices, is preferably used as an element having the opposite polarity, if possible. However, boron (B) has a segregation coefficient of about 0.78, which is larger than the segregation coefficient of phosphorus (P), and thus cannot be used in the above techniques.
As means for solving this point, Patent Document 3 discloses a method in which boron (B) is continuously added relative to phosphorus (P) of the primary dopant. This method enables production of an n-type crystal having an improved axial resistivity distribution by counter doping using phosphorus (P) as the primary dopant and boron (B) as the secondary dopant.